Add Playwright E2E test suite (25 tests across 4 specs)

- playwright.config.ts: headless CI setup with JSON/HTML reporters
- e2e/global-setup.ts: app reachability check before tests
- e2e/auth.spec.ts: 6 auth tests (sign-in, session, protected routes)
- e2e/compare.spec.ts: 8 API tests (validation, SSE flow, DB persistence)
- e2e/comparisons.spec.ts: 6 CRUD tests (list, detail, 404, view counts)
- e2e/user.spec.ts: 5 user tests (stats, pagination, session binding)
- e2e/helpers.ts: shared SSE parsing, auth, URL resolution utilities
- e2e/README.md: setup and run instructions
- AGENTS.md: updated with LLM provider notes and testing docs
- .gitignore: added playwright report/result directories
- package.json: added @playwright/test and test:e2e scripts

e2e/helpers.ts

@@ -0,0 +1,53 @@
+import { type Page, type Request, type Response, expect } from "@playwright/test";
+import { baseURL, targetHost, testUser } from "../playwright.config";
+
+/** Resolve URL relative to the target host (bypasses localhost resolution). */
+export function resolveUrl(path: string): string {
+  const base = baseURL.replace("localhost", targetHost);
+  return `${base}${path.startsWith("/") ? path : "/" + path}`;
+}
+
+/** Sign in as the test user via the API and return the session cookie. */
+export async function getSessionCookie(page: Page): Promise<string> {
+  const res = await page.request.post(resolveUrl("/api/auth/sign-in/email"), {
+    data: { email: testUser.email, password: testUser.password },
+    headers: { "Content-Type": "application/json" },
+  });
+  if (!res.ok()) {
+    throw new Error(`Sign-in failed: ${res.status()} ${await res.text()}`);
+  }
+  const setCookie = res.headers()["set-cookie"] ?? "";
+  const match = setCookie.match(/session_token=([^;]+)/);
+  if (!match) throw new Error("No session_token cookie in sign-in response");
+  return `session_token=${match[1]}`;
+}
+
+/** Parse SSE stream from a fetch response body. */
+export async function parseSSEStream(body: string): Promise<Record<string, unknown>[]> {
+  const events: Record<string, unknown>[] = [];
+  for (const line of body.split("\n")) {
+    const trimmed = line.trim();
+    if (!trimmed || trimmed.startsWith("event:")) continue;
+    if (trimmed.startsWith("data:")) {
+      const dataStr = trimmed.slice(5).trim();
+      try {
+        events.push(JSON.parse(dataStr));
+      } catch {
+        // skip malformed
+      }
+    }
+  }
+  return events;
+}
+
+/** Extract the last SSE event of a given type. */
+export function lastSSEOfType(
+  events: Record<string, unknown>[],
+  type: "progress" | "done"
+): Record<string, unknown> | undefined {
+  return events.reverse().find((e) => e[type] !== undefined);
+}
+
+// Re-export common helpers
+export { expect, resolveUrl as url, testUser };
+export type { Page, Request, Response };